Heat and Mass Transfer - Set 04 - ObjectiveBooks

# Practice Test: Question Set - 04

1. Thermal conductivity of glass wool varies from sample to sample because of variation in
(A) Composition
(B) Density
(C) Porosity
(D) All of the above

2. According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
(A) Absolute temperature
(B) Square of the absolute temperature
(C) Cube of the absolute temperature
(D) Fourth power of the absolute temperature

3. Thermal conductivity of a material may be defined as the
(A) Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
(B) Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
(C) Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
(D) All of the above

4. In case of liquids and gases, the heat transfer takes place according to
(A) Conduction
(B) Convection
(D) None of these

5. Which of the following has maximum value of thermal conductivity?
(A) Aluminium
(B) Steel
(C) Brass
(D) Copper

6. The logarithmic mean temperature difference (tm) is given by (where Î”t1 and Î”t2 are temperature differences between the hot and cold fluids at entrance and exit)
(A) tm = (Î”t1 - Î”t2)/ loge (Î”t1/Î”t2)
(B) tm = loge (Î”t1/Î”t2)/ (Î”t1 - Î”t2)
(C) tm = tm = (Î”t1 - Î”t2) loge (Î”t1/Î”t2)
(D) tm = loge (Î”t1 - Î”t2)/ Î”t1/Î”t2

7. Moisture would find its way into insulation by vapour pressure unless it is prevented by
(A) High thickness of insulation
(B) High vapour pressure
(C) Less thermal conductivity insulator
(D) A vapour seal

8. The radiation emitted by a black body is known as
(D) All of these

9. Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in
(A) Electric heater
(B) Steam condenser
(C) Boiler
(D) Refrigerator condenser coils

10. According to Newton's law of cooling, the heat transfer from a hot body to a cold body is
(A) Directly proportional to the surface area
(B) Directly proportional to the difference of temperatures between the two bodies
(C) Either (A) or (B)
(D) Both (A) and (B)

11. According to Prevost theory of heat exchange
(A) It is impossible to transfer heat from low temperature source to t high temperature source
(B) Heat transfer by radiation requires no medium
(C) All bodies above absolute zero emit radiation
(D) Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

12. Reynolds number is the ratio of
(A) Energy transferred by convection to that by conduction
(B) Kinematic viscosity to thermal diffusivity
(C) Inertia force to viscous force
(D) None of the above

13. Heat transfer by radiation mainly depends upon
(A) Its temperature
(B) Nature of the body
(C) Kind and extent of its surface
(D) All of the above

14. In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.
(A) Same
(B) Less
(C) Greater
(D) None of these

15. Thermal diffusivity is
(A) A dimensionless parameter
(B) Function of temperature
(C) Used as mathematical model
(D) A physical property of the material

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